Antenna arrangement with at least one antenna, especially on the screen of a motor vehicle

ABSTRACT

An antenna system is described having at least one LMK antenna which is arranged on a window or another non-conducting part of a motor vehicle and has an antenna capacitance C a , and an antenna booster for the LMK reception. The antenna is characterized in that the distance of the antenna conductor(s) to the parts surrounding the LMK antenna is selected in such a way that the value of the harmful capacitance C a  of the LMK antenna, which is essentially formed by the capacitance between the antenna conductors and the conductive parts surrounding the antenna, is selected to be greater than the value at which the maximum antenna output voltage results, and the value of the harmful capacitance C a  is selected such that, by increasing the source capacitance of the antenna booster, the signal-to-noise ratio is optimized in the lower LMK range.

FIELD OF THE INVENTION

The present invent ion relates to an antenna system having at least oneLMK (Long-Medium-Short) radio aerial antenna which is arranged on awindow or another non-conducting part of a motor vehicle and has anantenna capacitance C_(a), and having an antenna booster for the LMKreception.

Such antenna systems are used in particular on the rear window of amotor vehicle. However, they can of course be used on other windows suchas side windows, the windshield or other non-conducting parts of motorvehicles such as spoilers.

BACKGROUND INFORMATION

An antenna system is described in European Published Patent ApplicationNo. 0 155 647. This known antenna system, which obviously is conceivedexclusively for use on the rear window of motor vehicles, is arranged onan area not covered by the heating field. The LMK antenna has a planarextension.

It can be inferred from the teaching of European Published PatentApplication No. 0 155 647 that the size of the antenna area must be in aspecific relationship to the size of the available area, in order toreceive a maximum input signal for the antenna booster. It isspecifically explained that, in order for the reception to becomeoptimum, it is also absolutely necessary that the signal voltage bemaximum at the input of the LMK booster.

In this context, it is apparently assumed that the noise voltage at theinput terminal of the LMK booster is independent of the sourceimpedance.

However, in practice it has turned out that when using an antenna systemaccording to European Published Patent Application No. 0 155 647, thereception is not optimal under certain conditions.

SUMMARY OF THE INVENTION

According to the present invention, it has been recognized that this isattributable to the fact that a maximization of the signal voltage atthe input terminal of the antenna booster does not deliver an optimalresult under all operating conditions, since the assumption, apparentlymade in European Published Patent Application No. 0 155 647, about theindependence of the noise voltage from the source impedance isincorrect. Namely, the noise voltage of an FET (field-effect transistor)customarily used as input transistor in the LMK booster actuallyincreases perceptibly, at least in the long-wave range and in the “low”medium-wave range, when a conductive surface acting as antenna is usedas the signal source.

The reason for this is that the conductive surface acts as a capacitorwhose impedance increases as the frequency falls. All transistorsexhibit minimal noise only in response to a quite specific sourceimpedance. Their noise increases in the case of impedance deviatingtherefrom. Specifically, this means that although the antenna known fromEuropean Published Patent Application No. 0 155 647 may be optimallymatched in the upper medium-wave range and in the short-wave range (alsoknown as the upper LMK range), losses in the reception quality areaccepted in the lower medium-wave range and in the long-wave range(lower LMK range). This is also of particular disadvantage since thesignal-to-noise ratio of the antenna is markedly poorer anyway in thelower LMK range and, above all, in the long-wave range, than in theupper LMK range.

An object of the present invention is to further develop an antennasystem in such a way that optimum reception is yielded under virtuallyall operating conditions and frequencies.

In this context, the present invention starts out from the root idea ofcarrying out the matching between the LMK antenna and the antennabooster in such a way that an optimal signal-to-noise ratio results inthe lower LMK range.

To that end, according to the present invention, the distance of theantenna conductor(s) to the parts surrounding the LMK antenna isselected in such a way that the value of the harmful capacitance C_(a)of the LMK antenna, which is essentially formed by the capacitancebetween the antenna conductors and the conductive parts surrounding theantenna, is selected to be greater than the value at which the maximumantenna output voltage results, and that the value of the harmfulcapacitance C_(a) is selected such that, by appropriate selection of thesource impedance of the antenna booster, the signal-to-noise ratio isoptimized not in the upper, but rather in the lower LMK range.

Thus, the present invention breaks with the usual procedure, accordingto which harmful or stray capacitance is minimized to the greatestextent possible. On the contrary, according to the present invention, acomparatively large harmful capacitance is deliberately “adjusted”, bywhich the source capacitance of the input transistor is increased insuch a way that, although the input signal is not maximal, thesignal-to-noise ratio is probably optimal.

This matching according to the present invention can be achieved, forexample, by selecting conductive area A acting as antenna to be largerthan is described in European Published Patent Application No. 0 155647. Since according to the present invention, area A is enlargedcompared to the related art, the capacitance of the antenna increasesspecific to the electromagnetic field.

At the same time, the harmful capacitance increases because of thereduced distance to the conductive parts such as metallic body parts,the heating wires of the rear window, etc. surrounding the antennaconductors. Therefore, given the same frequency f of the electromagneticsignal, the amount of the capacitive portion Z_(k) of the sourceimpedance is smaller. Consequently, the amount Z of the source impedancedecreases.

It may be that the signal voltage at the booster input also decreases,however, the input noise of the transistor utilized is also verymarkedly reduced in the lower LMK range at the same time. In thelong-wave range (with small signal frequency f) and in the lowermedium-wave range, this measure leads to an improvement in thesignal-to-noise ratio for virtually all common transistors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the noise voltage of a customary LMK booster having an FETJ310 as the boosting element.

FIG. 2 shows schematically a rear-window antenna.

FIG. 3 shows measurements for clarifying the present invention.

FIG. 4 shows the dependence of the antenna capacitance on therelationship b/h.

DETAILED DESCRIPTION

FIG. 1 shows the noise voltage of a customary LMK booster, having an FETJ310 as boosting element, as functions of the frequency in the long-waveand medium-wave range. The frequency in MHz is plotted on the abscissa,while the noise voltage is indicated on the ordinate. FIG. 1 indicatesthe dependence of the noise voltage as a function of the frequency fordifferent source impedances. Reference is made to the legend in FIG. 1for the values of the source impedances. It can be gathered from FIG. 1that, with diminishing source impedance, the noise voltage for thetransistor presented by way of example increases sharply, at least forfrequencies under 1 MHz.

FIG. 2 shows by way of example the arrangement of an LMK antenna 1 ofthe present invention on the rear window of a motor and an antennabooster for an LMK reception 1 a. In this case, LMK antenna 1 isdisposed between a heating field 2 and a conductive, frame 3 of the rearwindow. Centered in this area is the antenna as a conductive area. Theantenna has transverse measurement b and distance a from frame 3 andheating field 2, respectively. c is the distance of the antenna from thelateral boundary of the window. In one embodiment, antenna booster 1 aincludes an input transistor corresponding to an FET J-Transistor.

The distance between the topmost conductor of heating field 2 and frame3 of the window is designated by h.

In the exemplary embodiment presented in greater detail below, withoutrestriction of the generality of the present invention:

h=13 cm

To optimize the signal-to-noise ratio, lateral distance c of the antennato the conductive window frame has been retained constantly at 3 cm.Only transverse measurement b is varied. However, it is of course alsopossible to vary both c and b.

FIG. 3 shows the measuring results obtained with a car radio selected asan example. The relationship b/h is plotted on the abscissa, while theantenna voltage is plotted on the left ordinate and the sensitivity fora signal-to-noise ratio of 20 dB is plotted on the right ordinate.

The values for various frequencies are specified, the allocation of theutilized symbols to specific frequency values being indicated in thelegend below the diagram.

FIG. 3 shows that a maximum antenna voltage is indeed obtained at avalue of the relationship b/h of 0.4, however an optimal signal-to-noiseratio is not obtained, at least for frequencies in the lower LMK range.

On the other hand, if—as is proposed according to the present inventionby way of example for the value h=13 cm—a value of the relationship b/hof 0.6 is selected, then a lower antenna voltage is obtained in theentire frequency range, which means non-linear distortions are reducedto the same degree. At the same time, the signal-to-noise ratio improvesin the lower LMK range, while it is only insignificantly reduced in theupper LMK range.

FIG. 4 shows the capacitance of the antenna in pF as a function of therelationship b/h. As FIG. 4 shows, the capacitance rises in a largelylinear manner up to a value b/h=0.6. Above this value, the harmfulcapacitance C_(a) rises disproportionately given the considered height hof the AM antenna, which means not only the antenna voltage, but alsothe signal-to-noise ratio diminishes.

The present invention has been described above by way of example withoutrestricting the general inventive idea. Thus, widely differing changesare possible compared to the exemplary embodiment shown. A few of theseare indicated—not conclusively—in the following:

In particular, the value 0.6 for an optimal b/h relationship accordingto the teaching of the present invention is valid only for a height h ofthe available area of 13 cm. When working with other heights h, theoptimal b/h relationship is at different values; these values can befound on the basis of the above explanations without inventiveassistance, in that the signal-to-noise ratio is optimized in the lowerLMK range. This always leads to a greater b/h relationship than theoptimization of the signal-to-noise ratio for the upper LMK range or themaximization of the signal magnitude for the upper LMK range.

Furthermore, it is possible to apply the antenna on any windows—not onlythe rear window—or on other non-conductive parts of the vehicle such asspoilers, built-on parts made of a plastic material, etc. In thearrangement of the antenna on the rear window, it is also possible tointegrate into the entire antenna system a VHF antenna, which uses theheating wires of the heatable rear window. The LMK antenna of thepresent invention can, of course, also have a form different from thatshown, for example, a non-rectangular form or the shape of a U or an E.

In addition, it is possible to provide antennas of the present inventionin the rear window both above and below the region used for the heatingwires.

What is claimed is:
 1. An antenna system, comprising: at least one LMKantenna having an antenna capacitance and arranged on one of a windowand another non-conductive part of a motor vehicle, the at least one LMKantenna including at least one antenna conductor; and an antenna boosterfor an LMK reception, wherein: an antenna output voltage passes througha maximum as a function of a relative width of the at least one LMKantenna, the antenna capacitance increases in accordance with anincrease of the relative width of the at least one LMK antenna, adistance of the at least one antenna conductor to parts surrounding theat least one LMK antenna is selected such that a value of the antennacapacitance of the at least one LMK antenna is selected to be greaterthan a value at which the maximum of the antenna output voltage results,and the value of the antenna capacitance is selected such that,resulting at an input of the antenna booster is a capacitance at which asignal-to-noise ratio of the antenna booster is greater in a lower LMKrange than if the maximum of the antenna output voltage is present. 2.The antenna system according to claim 1, wherein: the antenna boosterincludes an input transistor corresponding to an FET J-transistor. 3.The antenna system according to claim 1, wherein: the at least oneantenna conductor is rectangular and planar.
 4. The antenna systemaccording to claim 3, wherein: the at least one antenna is formed as oneof a broad strip and a grid.
 5. The antenna system according to claim 1,wherein: the at least one conductor includes a plurality of conductorsthat are parallel to one another and are arranged as a wire structurethat forms the at least one LMK antenna, and a connection point isarranged on a narrow side of the wire structure.
 6. The antenna systemaccording to claim 5, wherein: the plurality of conductors areinterconnected in a non-electroconductive manner on a side opposite theconnection point.
 7. The antenna system according to claim 5, wherein:the plurality of conductors are imprinted on the window.
 8. The antennasystem according to claim 1, wherein: the at least one LMK antenna isnot connected directly to a heating field arranged in the window.
 9. Theantenna system according to claim 8, wherein: the heating field is usedfor a VHF reception.
 10. The antenna system according to claim 1,wherein: the value of the antenna capacitance is as a function of therelative width of the at least one LMK antenna, and a height of apredefined free surface area for the at least one LMK antenna is derivedfrom a sum of a distance of the at least one antenna conductor from anupper boundary of the window, a distance of the at least one antennaconductor from one of a heating field and a lower boundary of thewindow, and a transverse measurement of the at least one antennaconductor.
 11. The antenna system according to claim 10, wherein: avalue of the relative width of the at least one LMK antenna differs byat least ±20% from a value at which a maximum signal voltage is presentat the input of the antenna booster.
 12. The antenna system according toclaim 1, wherein: a distance of the at least one antenna conductor froman upper boundary of the window is substantially equal to a distance ofthe at least one antenna conductor from a heating field.
 13. The antennasystem according to claim 1, wherein: distances of the at least oneantenna conductor from a lateral boundary of the window aresubstantially equal.